The invention relates to an electric drive unit for electric motors in which logic components as well as power controlling semi-conductors are carried on a circuit board with surface leads, and two or more power distribution bars are disposed across the circuit board, each having a side edge provided with a number of integrally formed connector pins which are to be received in apertures in the circuit board for communication with the semi-conductors via the surface leads on the circuit board.
One problem concerned with previously known drive units of the above type relates to thermal stresses occurring between the power distribution bars and the circuit board during operation of the drive unit. Since the drive unit is intended to deliver electric power of several hundred amperes, there is generated a considerable heat in the drive components. The differences in thermal expansion/contraction properties of the power distribution bars and the circuit board result in severe stresses in the contact points between these two members as well as within the entire structure. An important factor in this matter is that the power distribution bars have to have considerable cross sectional areas to be able to communicate high magnitude currents without too high losses, and to meet these requirements prior art power distribution bars have been formed as rather massive members with integral rigid connector pins. Accordingly, temperature related dimension changes of the power distribution bars have resulted in undesirable stresses between the circuit board and the connector pins. After many temperature changing cycles this may result in fatigue damages and impaired soldering connections between the connector pins and the circuit board leads.
Another problem concerned with prior art drive units of the above type is the difficulty to accomplish a proper soldering of the connector pins to the circuit board leads, because the large dimensions of the power distribution bars and the integral connector pins have very efficient heat conducting properties, which means that there has to be applied a considerable amount of heat to the soldering spots to accomplish a proper soldering. This means in turn that other parts and components of the drive unit are also exposed to undesirably high temperatures during the soldering process. It also means that the heating costs for the soldering process have been unnecessarily high since it requires special heating equipment.
Still another drawback of prior art drive units of the above type relates to assembly problems resulting from the arrangement of a number of ferrite rings which are mounted loosely around the connector pins and which easily falls off the connector pins during the assembly operation. This necessitates specially adapted assembly procedures and causes, despite that, frequent disturbances in the assembly operations.
The main object of the invention is to accomplish a drive unit for electric motors, wherein the power distribution bars have increased flexibility so as to reduce thermal related stresses in the connector pin connections to the circuit board leads and components carried thereon, thereby facilitating assembly and increasing the service life of the drive unit.
Further objects and advantages of the invention will appear from the following specification and claims.
A preferred embodiment of the invention will be described in detail below with reference to the accompanying drawings.